How does that work?

Extending Brexit is a job for EU leaders, say numerous diplomatic sources. The EU’s 27 heads of state and government would have to decide unanimously at an EU summit on Thursday 21 March. But first the UK has to ask. The EU cannot consider the question until the British government makes a formal request to extend article 50.

Would the EU say yes?

Probably. While any single country has the right to block a Brexit extension, most diplomats think the EU would agree, although this cannot be taken for granted.

The wildcard is that EU leaders have never discussed the issue and often take a stricter line than officials. In December, for example, EU leaders decided it would be pointless granting the UK further legal assurances on the Irish backstop, concluding that another legal paper was unlikely to sway MPs in favour of a Brexit treaty. It turned out they were right. But blocking an extension could be seen as tantamount to forcing the UK to leave the EU without a deal. The EU does not want to go down in history with the blame for Brexit.

What is a ‘credible justification’?

That’s not entirely clear. The European parliament’s Brexit co-ordinator, Guy Verhofstadt, has said he opposes “any extension of article 50, even just for 24 hours, if it is not based on a clear majority from the House of Commons in favour of something”. Some EU sources say “credible justification” means time to hold a general election or a referendum. Others have no fixed view, and member states don’t want to be boxed in with strict criteria.

How long?

A short “technical” extension of two to three months to allow parliament to pass Brexit legislation would have been easy to agree if MPs had voted for the Brexit withdrawal agreement.

Now the deal has gone down in flames, the EU faces a dilemma. A short extension is seen as heightening the chances of the UK tumbling out of the EU just before European elections. But a long extension means the EU could be bogged down in Brexit for months or years, while numerous foreign and economic policy problems are jostling for attention.

Many see a short extension as pointless. “The problem is that after the massive defeat of [Tuesday] it is hard to believe that we can fix the deal if she gets three more sentences on the backstop,” a senior source said.

Various options have been mooted, from three to 21 months, but there is no fixed view.

What do EU players think?

Germany does not want to rush the UK out of the door and thinks the Brexit debate would benefit from breathing space. France also wants to keep options open, despite tough talk in public from Emmanuel Macron. Some traditional allies, such as the Netherlands, have voiced impatience. Some in the EU institutions are worried about Brexit “polluting” the EU’s agenda for months, with no upside.

Would there be strings attached?

Brexiters have raised the prospect of Brussels attaching tough conditions to any Brexit extension, but the EU would be bound by its own laws. The UK would remain an EU member state with the rights and obligations that entails, including annual payments to the EU budget, upholding judgments from the European court of justice and taking part in European elections.

So Nigel Farage could remain an MEP?

EU officials say the UK must be represented in the European parliament if it is a member state. Officials are incredulous that the British government would be ready to countenance “a blatant treaty violation of the rights of their citizens”, in the words of one source.

British citizens and EU citizens living in the UK could go to the European court to challenge the absence of British MEPs or laws from a parliament not representing them.

So the former Ukip leader could run again in European elections – along with the UK’s 72 other MEPs, who largely get far less attention.

Some British politicians think there is a quick fix: for example, a treaty change that would allow the government to appoint MEPs for a limited period. But the last EU treaty amendment – a eurozone crisis amendment in 2011 – took two and a half years to be ratified by the EU’s 28 parliaments. Senior officials have concluded this option is not realistic.

What happens now?

If MPs vote for more time, triggering a request from the government, diplomatic activity will intensify. Tusk will meet Angela Merkel, Emmanuel Macron, Mark Rutte, the Dutch prime minister, and Leo Varadkar, the Irish prime minister, over the coming days, in what will be crucial encounters in influencing the outcome.

But this high-wire decision is unlikely to be pre-cooked. The final judgment will be made by EU leaders next Thursday – a mere eight days before the Brexit sands run out.